1. FIELD OF THE INVENTION
The invention relates to a method for the preparation of .alpha.,.alpha.-dimethyl-4-methylene-cyclohexanemethanol (referred to hereafter as delta-terpineol), cis-beta-terpineol, and intermediates thereof.
2. BRIEF DESCRIPTION OF THE PRIOR ACT
Delta-Terpineol is a component of a number of essential oils, including juniper, sage, rosemary, and Scotch spearmint. Nevertheless, it is the least-known and most difficult to obtain of the four isomeric terpene alcohols known as terpineols. These four alcohols result from the hydration of turpentine or its major constituent, alpha-pinene or from the dehydration of terpin hydrate, as shown in Scheme I. ##STR1##
Mixtures of these alcohols and various terpene hydrocarbons are called "pine oils" and are major items of commerce. A typical pine oil from turpentine consists mainly of alpha-terpineol (I) with about 3% cis-beta-terpineol (IIa), 1% trans-beta-terpineol (IIb), and 0.5% delta-terpineol (V). Dehydration of terpin hydrate affords a pine oil with about 25% cis-and trans-beta-terpineols, about 16% gamma-terpineol (III), and about 1% delta-terpineol. Thus, of the four terpineols, only the alpha-isomer is available in quantity and in good purity. Beta-terpineol of up to 60-70% purity can be obtained by careful fractionation of the terpin dehydration reaction mixture but it invariably contains 20-30% of 3-terpinen-1-ol which has nearly the same boiling point as the cis-beta-isomer. Delta-terpineol cannot be obtained in good purity in this fashion because it occurs in such a low concentration in the crude terpineols mixture and has nearly the same boiling point as the trans-beta-isomer.
Development of the use of delta-terpineol in fragrances and synthetic essential oils has not been possible due to the lack of a commercially viable preparation. It has been synthesized by three laboratory routes [J. Indian Chem. Soc., 44 (5), 388-390 (1967); J. Org. Chem., 31, 2022 (1966); and J. Amer. Chem. Soc., 101, 3340 (1979)], but two require exotic starting materials and expensive reagents. Only one preparation uses commerical turpentine as the ultimate feedstock but the overall yield of this multi-step procedure is less than 2 molar %.
The procedure of the present invention has three major advantages over the prior art. First, it is a single step process. Second, it proceeds in good selectivity to the desired delta-isomer; little or no trans-beta-terpineol or terpinen-1-ol is produced. Third, it uses 1,8-Cineole (eucalyptol) as its starting material. 1,8-cineole is widely available from natural sources, especially the essential oil of Eucalyptus globulus and from turpentine as a by-product of commercial pine oil production. The procedure of the invention has a further advantage in that it produces high purity cis-beta-terpineol as the only major co-product.
Cleavage of the ether bond of 1,8-cineole is well known to occur in the presence of acids. This reaction is very similar to the reactions of Scheme I and the product of such cleavage is again largely alpha-terpineol. Base-induced isomerization of 1,8-cineole has been attempted [J. Sci. Hiroshima Univ., Ser.A, 18 (2), (1954)] but no reaction was observed even after 60 hours at 160.degree. C. Treatment with sodium peroxide did succeed in isomerizing 1,8-cineole, but the conversion was at best 20% and the only product observed was the alpha-terpineol isomer.